1) Field of the Invention
The present invention relates to a signal incoming direction estimation apparatus and, more particularly, to a technique suitable for use in a wireless system for detecting the incoming angle of a signal (electromagnetic wave) using a sensor array.
2) Description of Related Art
FIG. 7 shows a block diagram of an apparatus for estimating the incoming direction of electromagnetic waves as an example of a signal incoming direction estimation apparatus. The electromagnetic wave incoming direction estimation apparatus shown in FIG. 7 employs a uniformly-spaced linear array antenna (hereinafter, abbreviated to ULA, occasionally) 100 the number of sensor elements (antennas) of which is NA (in this example, NA=9) and the intervals of antennas of which a red as a receiving system, and estimates the incoming direction of electromagnetic waves by utilizing the fast Fourier transform (FFT). In other words, it is an FMCW (Frequency Modulated Continuous Wave) radar apparatus for detecting the azimuth of a target object by frequency-converting a beat signal obtained by mixing a transmitted signal and a received signal and carrying out a beam scan to the converted signal.
The FMCW radar apparatus comprises, as its essential parts, the ULA 100 having NA elements (antennas) 101 arranged at uniform intervals (d), an RF down-converter 200 for obtaining a beat signal in the baseband frequency band by mixing a signal (RF signal) received by the ULA 100 and a transmitted RF signal and frequency-converting (down converting) it, a low pass filter (LPF) 300, and an A/D converter 400, and a CPU 500 as a receiving system, and at the same time, comprising a baseband oscillator (triangle wave oscillator) 600, an up-converter 700 for frequency-converting (up converting) a signal at the oscillation frequency (baseband frequency) of the oscillator 600 into an RF signal, and a transmitting antenna 800 as a transmitting system.
The ULA 100 is provided with switches 102, 103, 104, and 105 and by these switches 102, 103, 104, and 105, the antenna 101 to receive a signal (to input the received signal into the RF down-converter 200) is switched to another in a time division manner as will be described later. The RF down-converter 200 has a low noise amplifier (LNA) 201 and a mixer 202 and the received signal from the ULA 100 is, after low-noise amplification by the LNA 201, mixed by the mixer 202 with a signal from an RF voltage control oscillator 701 (that is, the transmitting RF signal) shared by the up-converter 700 and thus a beat signal in the baseband frequency band, that is, a signal having a delay time, a Doppler frequency shift, and a beat frequency determined by the relative distance and the relative velocity between the present radar apparatus and the target object can be obtained.
Further, in the transmitting system, the up-converter 700 comprises the above-mentioned RF voltage control oscillator (RF-VCO) 701 driven by the output voltage signal (modulation control signal) of the baseband oscillator 600 and outputting an RF signal (transmission modulation signal), a high power amplifier (HPA) 703 for amplifying the output of the RF voltage control oscillator 701 to a predetermined signal level in accordance with the sensing distance, and a hybrid circuit (HYB) 702 for branching the output of the RF voltage control oscillator 701 into two and supplying one to the HPA 703 and the other to the mixer 202 of the RF down-converter 200.
Next, the operation of the radar apparatus constructed as described above, particularly, the operation of the receiving system is explained below. In the following explanation, each antenna 101 shown in FIG. 7 is denoted by symbol Ak (k=1 to NA) where Al denotes the nearest antenna 101 to the transmitting antenna 800 (letting position of the antenna 101 be the origin) among the antennas 101 constituting the receiving ULA 100 shown in FIG. 7.
First, if it is assumed that Ns of independent signals xm (t) arrive at the ULA 100 with angles θm (m=1 to Ns) at time t, the phase difference between the signals xm (t) arriving at the antenna Al and the antenna Ak is expressed by the following expression (1.1) with the origin being the phase reference.
                              ϕ          m          k                =                                            2              ⁢              π                        λ                    ⁢                      (                          k              -              1                        )                    ⁢          d          ⁢                                          ⁢                      sin            ⁡                          (                              θ                m                            )                                                          (        1.1        )            
Therefore, the output vk (t) from the antenna Ak is expressed, along with the noise signal nk (t), by the following expression (1.2).
                                          v            k                    ⁡                      (            t            )                          =                                            ∑                              m                =                1                                            N                s                                      ⁢                                                  ⁢                                                            x                  m                                ⁡                                  (                  t                  )                                            ⁢                              exp                ⁡                                  (                                      jϕ                    m                    k                                    )                                                              +                                    n              k                        ⁡                          (              t              )                                                          (        1.2        )            
However, it is assume that each signal is a baseband representation and that the direction of an incoming angle is positive if the direction is in the clockwise direction with respect to the Y-axis (0°).
Next, it is assumed that the period of the signal xm (t) is Tm, NT of samples taken in from each of the antennas Ak during the period Tm are vectorized into vk as shown in the following expression (1.3), and the Fourier transform thereof is Vk as shown in the following expression (1.4).vk=[vk(l) . . . vk(NT)]  (1.3)Vk=[Vk(l) . . . Vk(NT)]  (1.4)
However, Vk (l) (l=1 to NT) is expressed by the following expression (1.5) where the Fourier transform coefficients of xm (t) and nk (t) are Xm (l) and Nk (l), respectively.
                                          V            k                    ⁡                      (            1            )                          =                                            ∑                              m                =                1                                            N                s                                      ⁢                                                  ⁢                                                            X                  m                                ⁡                                  (                  1                  )                                            ⁢                              exp                ⁡                                  (                                      jϕ                    k                    m                                    )                                                              +                                    N              k                        ⁡                          (              1              )                                                          (        1.5        )            
Finally, if Vk (l) multiplied by exp (−jφkq) and summed in terms of k is expressed by Vq (l), the following expression (1.6) is obtained,
                                                                                          V                  q                                ⁡                                  (                  1                  )                                            =                            ⁢                                                ∑                                      k                    =                    1                                                        N                    A                                                  ⁢                                                                  ⁢                                                                            V                      k                                        ⁡                                          (                      1                      )                                                        ⁢                                      exp                    ⁡                                          (                                              -                                                  jϕ                          k                          q                                                                    )                                                                                                                                              =                            ⁢                                                                    ∑                                          m                      =                      1                                                              N                      s                                                        ⁢                                                                          ⁢                                                                                    X                        m                                            ⁡                                              (                        1                        )                                                              ⁢                                          {                                                                        ∑                                                      k                            =                            1                                                                                N                            A                                                                          ⁢                                                                                                  ⁢                                                  exp                          ⁡                                                      [                                                          j                              ⁡                                                              (                                                                                                      ϕ                                    k                                    m                                                                    -                                                                      ϕ                                    k                                    q                                                                                                  )                                                                                      ]                                                                                              }                                                                      +                                                                                                      ⁢                                                ∑                                      k                    =                    1                                                        N                    A                                                  ⁢                                                                  ⁢                                                                            N                      k                                        ⁡                                          (                      1                      )                                                        ⁢                                      exp                    ⁡                                          (                                              -                                                                              j                            ⁢                            ϕ                                                    k                          q                                                                    )                                                                                                                              (        1.6        )            where the inside of the brackets { } of the above-mentioned expression (1.6) is an array factor for m-th (m=1 to Ns) signal expressed by the following expression (1.7), and it will be found that the in-phase condition is satisfied and there exists a peak when θq=θm.
                              ∑                      k            =            1                                N            A                          ⁢                                  ⁢                  exp          ⁢                      {                          j              ⁢                                                2                  ⁢                  π                                λ                            ⁢                              (                                  k                  -                  1                                )                            ⁢                              d                ⁡                                  [                                                            sin                      ⁡                                              (                                                  θ                          m                                                )                                                              -                                          sin                      ⁡                                              (                                                  θ                          q                                                )                                                                              ]                                                      }                                              (        1.7        )            
Therefore, if the above-mentioned expression (1.6) is calculated while changing l and θq, the position on the frequency index: l and the angle index: q of each signal source is known as the sum of the array factors weighted with the spectrum Xm (l) of the baseband signal. The calculation (digital signal processing) is carried out by the CPU 500 for a beat signal having passed from the RF down-converter 200 through the LPF 300 and having been digitized by the A/D converter 400.
Here, it is necessary to realize pointed directional beams using a number of sensor elements (antennas) 101 in order to separate signal sources closely adjacent angularly to each other with a radar apparatus (signal incoming direction estimation apparatus) using the ULA 100. In the frequency band for which the variations in the hardware performance and the cost cause no problem, the object can be attained by simply preparing the baseband generation circuits (the RF down-converter 200, the LPF 300, the A/D converter 400, etc., shown in FIG. 7) of the same number as that of the sensor elements, but in the frequency band for which the above-mentioned problems cannot be ignored (for example, a millimeter wave band such as a 76 GHz band), it is a mandatory item required by the system development to reduce the number of baseband generation circuits.
Then, in the signal incoming direction estimation apparatus in such a frequency band, as shown in FIG. 7, the above-mentioned switches 102, 103, 104, and 105 are arranged for each of the sensor elements 101 and these switches are switched at a certain timing to share the baseband generation circuit (the RF down-converter, the LPF 300, the A/D converter 400, etc.) in a time division manner and thereby the antenna beam is formed by the digital signal processing (this is referred also to as an electronic scan system). An example of this type of signal incoming direction estimation apparatus (switching beam former) is disclosed, for example, in the following Patent Document 1.
[Patent Document 1] Japanese Patent Laid-Open (Kokai) HEI 6-281715
However, in the above-described configuration, the following problems arise.
(1) As the number NA of sensor elements increases, the simultaneity of the signals obtained from each of the sensor elements 101 collapses and in particular, in such a case where the signal source is moving, there is the possibility that the original purpose of the apparatus to estimate the incoming direction cannot be attained.
(2) To realize the switches 102, 103, 104, and 105 whose losses are small and variations in characteristics is small will become more difficult as the frequency to be handled becomes higher. Particularly, the construction shown in FIG. 7 has a disadvantage because losses are caused to occur by the switches 102, 103, 104, and 105 at the previous stage of the LNA 201.
(3) Further, in order to avoid the electromagnetic interaction between the switches 102, 103, 104, and 105, it is necessary to widen the distance between the switches 102, 103, 104, and 105 and this will reduce the relative proportion of the area occupied by the functional components on the layout pattern of an IC (that is, the cost is raised).